The present invention relates to an oxygen sensor that is capable of measuring the concentration of oxygen in for example, the exhaust from an internal combustion engine.
With a view to improving fuel economy and reducing emissions, feedback control of the air/fuel ratio for internal combustion engines is currently performed on the basis of measurements of the oxygen partial pressure of the exhaust from the engine. The oxygen partial pressure of the exhaust gas is measured with an oxygen sensor that employs a layer of an oxygen-ion conductive electrolyte such as a solid solution compound of zirconia or yttria. An example of this type of oxygen sensor is proposed in Unexamined Published Japanese Utility Model Application No. 100658/1985. The sensor shown in this patent has an oxygen-ion conductive solid electrolyte layer disposed in the gap between an inner and an outer electrode that are concentrically arranged in a cylindrical form. This sensor may be considered as an oxygen concentration sensor which measures a gas to be analyzed on the outer surface of the solid electrolyte layer. That is, the sensor constitutes a measuring system without introducing a reference atmosphere.
In order to ensure that atmospheric air which is used as & a reference oxygen source in the sensor described above, is introduced to make contact with the inner surface of the solid electrolyte layer without mixing with the gas to be analyzed, a so-called "green sheet" is used. The green sheet has a slit that provides an air introducing channel and must be wrapped around a core material, which may be in a bar form and which is covered with the solid electrolyte layer. The assembly of the core and the green sheet is sintered to make a product having a channel for introducing the reference atmosphere. In other words, at least two layers one being a green sheet of the solid electrolyte layer for effecting measurement of the oxygen partial pressure and the other being a green sheet that serves as a spacer and is provided with the slit described above, must be sintered after they are concentrically wound. However, considerable difficulty is involved in producing a concentric assembly of more than one green sheet without introducing any distortion. Also the required increase in the number of steps in the manufacturing process leads to a lower production rate.
Another problem with the technique described above is that the use of a bar for the core material causes an increase in the heat capacity of the oxygen sensor. In order to hold the sensor at a temperature that is suitable for the intended measurement a heater for the electrodes must have a high heat generation rating, but then the power consumption of the heater is increased while its durability is reduced.
Unexamined Published Japanese Patent Applications Nos. 125448/1980 and 36949/1985 disclose oxygen sensors that determine the oxygen partial pressure with measuring electrodes formed on the surfaces of a detecting element having a flat plate form.
However such conventional oxygen sensors having measuring electrodes formed on the surfaces of a detecting element in flat plate form have inherent directionality with respect to the direction of flow of the gas to be analyzed, and hence their outputs vary depending upon the direction of flow of the & gas of interest or the position at which the sensor is installed, thereby making it difficult to achieve proper control of the A/F ratio and other parameters on the basis of the sensor output. In order to alleviate this problem, an oxygen sensor has been proposed in Japanese Patent Application No. 66663/1986 employing a tubular detecting element.